In a known steering system of this kind (U.S. Pat. No. 3,730,288 ), the main loop and the auxiliary loop are of identical construction. They work parallel to one another on a steering motor with two cross-connected cylinders. If one loop fails, the steering system is still capable of being operated with the aid of the other loop. However, both steering units have to be operated with the steering hand-wheel. The required moment is therefore about twice as high as it is with a single-loop steering system and steering is sluggish.
The invention is based on the problem of providing a two-loop steering system for motor vehicles of the kind mentioned in the introduction, in which sluggish steering is avoided whilst nevertheless ensuring that the steering system continues to operate safely when one loop fails.
This problem is solved according to the invention in that a change-over device connects the steering hand-wheel in normal operation to the main loop steering unit and in the event of a fault in the main loop connects it to the auxiliary loop steering unit, and each motor chamber of the steering motor is connected with the aid of a change-over valve in normal operation to a motor line of the main loop steering unit and in the event of a fault in the main loop to the associated motor line of the auxiliary loop steering unit, with the motor line of the main loop steering unit being isolated.
With this construction, in normal operation only the main loop is effective and in the event of a fault in the main loop (failure of the pump, line break and so on) only the auxiliary loop is effective. The consequence of this is that the operator needs to adjust only one steering unit using the steering handwheel, that is to say, steering is smooth, as usual. The change-over valve associated with each motor chamber ensures that in an emergency the main loop is completely separated from the auxiliary loop, and a line break in the main loop or other fault cannot have an adverse effect on the auxiliary loop or on the steering. It is a further advantage that in normal operation and in an emergency the same motor chambers are pressurized, so that there is no substantial difference in driving performance either. This also applies when two cross-connected cylinders are used as the steering motor. Alternatively, however, a single cylinder with two motor chambers can be used.
It is especially advantageous for the change-over valves to be arranged on the steering motor. This means that the motor lines of both steering units are guided as far as the steering motor, but there is no common conduit for the main loop and the auxiliary loop between the change-over valve and the steering motor. There are therefore no conduit sections that are common to both loops which in the event of a break would thus lead to a complete failure.
It is preferable for a control pressure line, which in the event of a fault in the main loop carries a different pressure from that in normal operation, to be provided, and for the change-over device and/or the change-over valve, of which there is at least one, to be operable in dependence on the control pressure. Such a control pressure line can readily be provided in a steering system. The control pressure can be used not only as a signal but also as an energy carrier. The change in pressure is a reliable signal for the occurrence of a fault.
In particular, the control pressure line can be connected by way of a throttle to the tank line of the main loop steering unit between the steering unit and a pressure-maintaining device, and also to both motor lines of the main loop steering unit by way of a respective non-return valve opening towards the motor line. Whereas the control pressure in normal operation assumes the value preset by the pressure-maintaining device, it falls to atmospheric pressure when there is a break in the pump line, the tank line or one of the motor lines, or when the pump fails.
The change-over device can be adjustable in particular mechanically by means of a piston biassed by the control pressure and in the opposing direction by a spring.
Similarly, the change-over valve, of which there is at least one, may comprise a piston slider biassed by the control pressure and in the opposing direction by a spring.
The change-over device is preferably a change-over gear. A small adjustment movement connects the driving gearwheel selectively to the one or other driven gearwheel. Change-over clutches and other change-over devices are possible alternatives.
The invention is explained in detail below with reference to a preferred embodiment illustrated in the drawing.